MENDELIAN GENETICS
1.
2.
3.
4.
Mendel’s work
Monohybrid inheritance and principal of segregation
Dihybrid inheritance and the principal of independent
assortment
Test cross
INTRODUCTION
• Explaining the mechanism of inheritance
• The mechanism relates to the numbers of
characteristic of inheritance
• The simple characteristic leads to the simpler
crossing over mechanism and ration
• This is followed by excluding the mutation
effects that will be discussed later (chapter 4)
GREGOR MENDEL
• Study in University
of Vienna
• His parents has a
small farm in
Austria
GREGOR MENDEL (cont)
• Austrian monk
• Studied the
inheritance of traits in
pea plants
• Developed the laws of
inheritance
• Mendel's work was
not recognized until
the turn of the 20th
century
GREGOR MENDEL (cont)
• Between 1856 and
1863, Mendel
cultivated and tested
some 28,000 pea
plants
• He found that the
plants' offspring
retained traits of the
parents
• Called the “Father of
Genetics"
MENDEL’S PEA PLANT TRAITS
GREGOR MENDEL (cont)
• Mendel stated that
physical traits are
inherited as
“particles”
• Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
GENETIC TERMINOLOGIES
• Character – heritable feature that varies among
individuals
• Trait – each variant for the character
• True-breeding – Plants homozygous for a
characteristic are true-breeding (Self-pollinate)
• Hybridization – mating or crossing over of two
true-breeding varieties
• P generation – parental generation/parent
• F1 generation – first filial generation (son)
• F2 generation – second filial generation
GENETIC TERMINOLOGIES (cont)
• Allele- alternate version of a gene
• Homozygote – pair of identical alleles for a character
• Heterozygote – two different alleles for a character
(Bb)
• Dominate allele – expressed in the heterozygote
• Recessive allele – not expressed in the heterozygote
• Homozygous dominant- BB
• Homozygous recessive - bb
• Genotype – genetic makeup
• Phenotype – appearance of an organism
TYPES OF GENETIC CROSS
1. Monohybrid cross - cross involving a single
trait
e.g. flower colour
2. Dihybrid cross - cross involving two traits
e.g. flower colour & plant height
PUNNET SQUARE
• Diagrammatic device for predicting the allele
composition of offspring from a cross between
individuals of known genetic makeup.
• 3 steps / generation = P gen, F1 gen, F2 gen
• Heterozygous allele - ?
• Homozygous allele - ?
• Phenotype - ?
• Genotype - ?
PUNNET SQUARE (cont)
• Can be used for monohybrid and also dihybrid
cross.
LAW OF INHERITANCE
Gregor Mendel introduce 2 laws
• Law of Segregation
• Law of Independent Assortment
LAW OF SEGREGATION
• Inherit only ONE characteristic @ Monohybrid
• Producing 3:1 of phenotypic inheritance
• Mendel use a large group of sample size to
explain this law
• Leads to a development of a model known as
Mendel’s Model
MENDEL MODEL
Four concepts in law of segregation
1. Alternative versions of genes account for variations in
inherited characteristics
2. For each character, an organism inherit two alleles,
one from each parent
3. If the two alleles at a locus differ, then one, the
dominant allele, determines the organism’s
appearance; the other, the recessive allele, has NO
noticeable effect in the organism’s appearance
4. The two alleles for a heritable character segregate
(separate) during gamete formation and end up in
different gametes
1 . ALTERNATIVE VERSIONS OF GENES
ACCOUNT FOR VARIATIONS IN
INHERITED CHARACTERISTICS
• Have 2 choices of alleles
• Existing in two version
• Depending on the
phenotypic or characteristic
derive in the genetic make
up
• Eg. Purple flower and white
flower
2. FOR EACH CHARACTER, AN
ORGANISM INHERIT TWO ALLELES,
ONE FROM EACH PARENT
• Each somatic cell in a
diploid organism has two
sets of chromosome
• Genetic locus represent
twice in diploid cell, once
in homolog of a specific
pair of chromosome
3. IF THE TWO ALLELES AT A LOCUS
DIFFER, THEN ONE, THE DOMINANT
ALLELE, DETERMINES THE ORGANISM’S
APPEARANCE; THE OTHER, THE
RECESSIVE ALLELE, HAS NO NOTICEABLE
EFFECT IN THE ORGANISM’S
APPEARANCE
• The plant have more purple colour due to its
dominant allele, vice versa
4. THE TWO ALLELES FOR A
HERITABLE CHARACTER SEGREGATE
(SEPARATE) DURING GAMETE
FORMATION AND END UP IN
DIFFERENT GAMETES
• An egg or sperm gets only one of the two
alleles that are present in the somatic cell of
the organism making the gamete
• The correspond depending on the types of
reproduction between meiosis and mitosis
• Further discussion after test cross
Example of
MONOHYBRID CROSS
P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR
x rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
P1 Monohybrid Cross Review
• Homozygous dominant x Homozygous
recessive
• Offspring all Heterozygous (hybrids)
• Offspring called F1 generation
• Genotypic & Phenotypic ratio is ALL ALIKE
F1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round r – Wrinkled
• Cross: Round seeds x Round seeds
•
Rr
x
Rr
R
R
r
RR
Rr
r
Genotype: RR, Rr, rr
Rr
Phenotype: Round &
wrinkled
rr
G.Ratio: 1:2:1
P.Ratio: 3:1
F1 Monohybrid Cross Review
• Heterozygous x heterozygous
• Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
• Offspring called F2 generation
• Genotypic ratio is 1:2:1
• Phenotypic Ratio is 3:1
HOW DOES THE PEAS LOOK LIKE?
•Genotypic Ratio
&
•Phenotypic Ratio
TEST YOURSELF!
1. Between blue flower, BB and yellow, yy
2. Between small leaf, ff and big leaf, Ff
LAW OF INDEPENDENT ASSORTMENT
• TWO characteristics at the same time @
Dihybrid cross
• Eg. Leaf colour and leaf size
• Using both dominant and recessive alleles in
each of the characteristics.
INDEPENDENT ASSORTMENT in
CHROMOSOME
• Mendel performed dihybrid crosses in plants that were
true-breeding for TWO traits.
• E.g a plant with green pod colour and yellow seed,
cross-pollinated with a plant that had yellow pod
colour and green seeds.
• Green pod colour = GG
• Yellow seed colour = YY
• Yellow pod colour = gg
• Green seed colour = yy
• The resulting F1 generation were all heterozygous for
green pod colour and yellow seeds (GgYy)
DIHYBRID CROSS
• Involves two pairs of contrasting traits
DIHYBRID CROSS
Round/Yellow:
Round/green:
wrinkled/Yellow:
wrinkled/green:
9
3
3
1
Phenotypic ratio 9:3:3:1
copyright cmassengale
36
DIHYBRID CROSS
• Traits: Seed shape & Seed colour
• Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
DIHYBRID CROSS
RY
RY
Ry
rY
ry
Ry
rY
ry
DIHYBRID CROSS
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
RY
ry
RrYy
Rryy
rrYy
rryy
Round/Yellow:
9
Round/green:
3
wrinkled/Yellow:
3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
HYPOTHESIS/CONCLUSION
• The alleles of seed colour and seed shape sort
into gametes independently of each other.
• Phenotypic ratio for IA = 9:3:3:1
TEST CROSS
• To determine if an individual exhibiting a
dominant trait is homozygous or heterozygous
for that trait.
• If all offspring display the dominant
phenotype, the individual in question is
homozygous dominant; if the offspring
display both dominant and recessive
phenotypes, then the individual is
heterozygous
TEST CROSS (cont)
• In some sources, the ‘test cross’ is defined as
being a type of backcross between the
recessive homozygote and F1 generation.
• F1 progeny are mated back to one of their
parents (or to individual with a genotype
identical to the parent)
• Backcross is often used synonymously with
testcross.
TEST CROSS
A mating between an individual of unknown genotype
and a homozygous recessive individual.
• Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
bc
b___
TEST CROSS
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
copyright cmassengale
bC
b___
c
bbCc
bbcc
44
If the plant being tested is
homozygous
If the plant being tested is
heterozygous
• G?W? X ggww
– (G=yellow; g=green; W=round; w=wrinkled)
– What will the expected phenotypic ratios be for
the above testcross?
SUMMARY of MENDEL’S LAW
LAW
PARENT CROSS
DOMINANCE / TrueTT x tt
breeding
tall x short
SEGREGATION
INDEPENDENT
ASSORTMENT
OFFSPRING
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green x
round & green
9/16 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green pods
1/16 wrinkled seeds & yellow pods